1. Field of the Invention
The present invention relates to a mask inspection apparatus and method.
2. Background Art
To form each pattern on a substrate, a reticle or photomask (hereinafter referred to as “mask”) is used in a manufacturing process of semiconductor devices. If the mask has a defect thereon, the defect is transferred onto the substrate. For that reason, a mask defect inspection needs to be carried out.
Examples of known mask inspection methods include a Die-to-Die inspection and a Die-to-Database inspection.
In the Die-to-Die inspection, optical images of the same pattern written at different positions of a mask are compared with each other. By contrast, in the Die-to-Database inspection, a reference image generated from design data (CAD data) used upon mask creation is compared with each of optical images of patterns written onto a mask.
In a mask inspection apparatus described in, for example, Japanese Patent Laid-open No. 2006-266864, the positions of a stage moved in X and Y directions in a state of a mask being held thereon are measured by laser interferometers. Each optical image is acquired using the results of measurement for comparison with a predetermined reference image.
As shown in FIG. 8 here, laser interferometers 112 and 114 respectively apply laser light to mirrors 111 and 113 provided at a stage 102 and receive light reflected by the mirrors 111 and 113 to thereby measure X-direction and Y-direction positions of the stage 102.
However, machining accuracy in polishing of each of these mirrors 111 and 113 is finite. Since the surfaces of the mirrors 111 and 113 are actually bent as shown in FIG. 8, positional displacements occur. Since a mask 101 supported by a support part 102a of the XY stage 102 is flexed by the influence of its gravity, for example, a positional displacement of Δx occurs in the X direction. When these positional displacements are combined together, an acquired optical image is distorted on the order of 20 nm to 30 nm as shown in FIG. 8. It was found that the distortion of the optical image is reproducible.
Since the reference image compared with each optical image is generated from the design data in the Die-to-Database inspection, such distortion as mentioned above does not occur. It was therefore difficult to perform a mask inspection with satisfactory accuracy.
Recently, miniaturization and higher densification of circuit patterns for a semiconductor device have been advanced and its resolution is approaching its limit. Therefore, a double patterning or double exposure technology for dividing a pattern into two masks and transferring high-density patterns using these two masks has been studied.
The registration or alignment of patterns for the two masks employed in the double patterning is required with a high accuracy of about 2 nm to 3 nm. Thus, when the masks used in the double patterning are inspected in particular, optical images obtained from the masks need not to have local distortion. However, the optical images are actually distorted on the order of 20 nm to 30 nm. It was thus difficult to satisfy this need.
The present invention has been made in view of the foregoing problems. That is, an object of the present invention is to provide a mask inspection apparatus and method capable of eliminating distortion of each optical image, which is caused by distortions of mirrors and flexure of a mask, and performing a mask inspection with satisfactory accuracy.